1. Field of the Invention
This invention relates to metal-coated substrates, and a method for their manufacture, as well as to composite material articles in which such metal-coated substrates, e.g., in the form of metal-coated fibers, may be used as reinforcing media.
2. Description of the Related Art
In a variety of applications, it is necessary and/or desirable to provide metal coatings, for example, thin films of nickel, gold, copper, silver, and the like, on various adhesion-resistant or poorly adhering substrates, such as glass and ceramics. Examples of such applications include conventional mirrors, conductive fillers, and reinforcing media.
Particularly in composite materials applications, for example in electromagnetic interference (EMI) shielding applications, where metal-coated glass fibers are employed as the discontinuous phase for reinforcement of continuous phase matrix materials (typically thermoplastic resins), it is critically important that the metal coating has a high degree of adhesion to the glass substrate.
Good adhesion of the metal coating is necessary in such applications in order that the metallic film be continuous in the final cured composite on the associated reinforcement elements (glass fibers).
Continuous metallic coatings are in turn essential in the aforementioned composite applications, since any discontinuities will lower the electrical conductivity of the composite. In general, the more highly conductive a composite is, the greater is its ability to provide effective EMI shielding.
In composite applications where the continuous phase matrix material is a thermosetting material, the presence of the metal-coated fibers therein may permit the matrix resin to be rapidly and uniformly cured by radio frequency or microwave heating of coating metal. It is apparent that the presence of discontinuities in the metal coatings on the substrate fibers will result in corresponding localized absences of the desired heating. This in turn will cause uneven curing rates in the composite, with concomitant localized stresses therin and decreased mechanical properties for the composite as a whole.
Further, the adhesion of the metal coating to the associated fiber substrate in composites applications must be satisfactory to withstand the abrading and shear to which the coated fibers are subjected in forming the composite.
Specifically, fiber-reinforced composites are typically made by dispersing the reinforcing fibers into the matrix resin with a mixing means such as a Brabender single screw extruder or a Werner-Pfleiderer twin screw extruder. During such dispersion, the metal coated fibers encounter substantial abrasion and shear via contact with one another. If the metal coating on the fibers is not strongly adherent in character, such contacts will propagate peeling, pitting, or other delamination of the metal coating, with the aforementioned deleterious consequences consequences on the performance and properties of the composite.
Japanese Kokai Tokkyo Koho 60/189105 discloses a conductive composite material, suitable for use in electromagnetic shielding applications, which consists of a thermoplastic resin, e.g., polystyrene, containing 5-40 weight percent of a conductive filler such as copper fibers, and 0.3-10 weight percent of a material for preventing the conductivity of the composite from deteriorating, such as glass fibers coated with tin, nickel, aluminum, or the like.
Japanese Kokai Tokkyo Koho 60/65179 discloses the use of nickel coated glass fibers in EMI shielding composites. The nickel coated fibers are prepared by soaking them in titanium sulfate, followed by calcining to form titanium dioxide coatings. The fibers next are soaked in aqueous stannous chloride-hydrochloric acid solutions, washed, activated in aqueous palladium chloride-hydrochloric acid solution to precipitate palladium on the fibers, and washed. Finally, the fibers are chemically plated with nickel using an aqueous solution of nickel sulfate, nickel citrate, sodium hypophosphate, sodium acetate, and ammonium chloride.
Japanese Kokai Tokkyo Koho 60/77151 describes electroless coating of glass fibers with conductive metal films, e.g., of nickel, copper, cobalt, iron, and alloys of nickel-copper, nickel-phosphorus, cobalt-phosphorus, cobalt-nickel-phosphorus, iron-nickel, and iron-cobalt, after sensitizing the glass substrate in an aqueous stannous chloride solution, and activated in an aqueous palladium chloride solution.
Spinning and subsequent metal coating of glass optical fiber preforms in vacuo is taught in Japanese Kokai Tokkyo Koho 57/156341. Disclosed metals include aluminum, nickel, and tin.
Japanese Kokai Tokkyo Koho 57/39284 discloses the electroless plating of glass fibers, after their immersion in an aqueous palladium nitrate solution and heat treatment, e.g., for 30 seconds at 400 degrees Centigrade. Nickel is mentioned as an example of the electrolessly plated metal.
Schladitz U.S. Pat. Nos. 4,096,823 and 4,097,624 disclose an apparatus and method for metallizing filaments such as glass, in a two-chamber system. In the first chamber, the filament drawn therethrough is impinged with plural streams of heated inert gas to "decontaminate", i.e., clean, itssurface. In the second chamber, the hot, decontaminated filament is impinged upon by a gaseous, thermally decomposable metal compound in plural streams, to thermally decompose the compound and metallize the filament.
A general review of composite materials containing conductive fibers is contained in "Conductive Polymeric Composites From Short Conductive Fibers" by Bigg, D.M., et al., Polym. Sci. Technol., 15 (Conduct. Polym.), 23-28, Battelle Columbus Laboratory, Columbus, Ohio (1981).
The techniques utilized in accordance with the abovediscussed references for metallizing glass substrates are all characterizable by various deficiencies, such as inadequate adhesion of the metal coating to the glass, and/or expensive, time-consuming, or complex processing requirements.
Accordingly, it would be a significant advance in the art to provide adherent metal coatings on adhesion-resistant substrates such as glasses, ceramics, and the like, in a readily achieved, simple, and inexpensive manner, whereby the resulting metal-coated substrate is particularly useful in applications such as composite materials manufacture, where delamination-producing abrasion and shear are imposed on metal-coated reinforcing media.
With respect to the use of polysilicate, titania, or alumina interlayers employed in the present invention to provide the advantages noted in the preceding paragraph, and with reference to the preferred method of the present invention wherein a sol gel dispersion is utilized to form such interlayers, related art includes: "Sol-Gel Transition in Simple Silicates", Brinker, C. J., et al., Journal of Non-Crystalline Solids, 48, 1982, pp. 47-64, North Holland Publishing Company; "Glassy and Crystalline Systems From Gels: Chemical Basis and Technical Application", Dislich, H., Journal of Non-Crystalline Solids, 57, 1983, pp. 371-388, North Holland Publishing Company; and "Glass Formation Through Hydrolysis of Si(OC.sub.2 H.sub.5).sub.4. With NH.sub.4 OH and HCl Solution", Nogami, M., et al., Journal of Non-Crystalline Solids, 37, 1980, pp. 191-201, North Holland Publishing Company.